The invention relates to a seal for a magnetisable shaft, which comprises a housing with a radially displaceable sealing lip of polymeric material and a preliminary seal on the side facing away from the space to be sealed.
A seal of this type is disclosed in the German Patent No. 33 05 649. A conical surface axially defining the sealing surface of the sealing lip, and facing away from the sealed space, includes with the shaft axis a steeper angle than the opposite conical surface that faces toward the sealed space. This arrangement produces a permanent pumping action in the direction of the sealed space. This pumping action has a positive sealing effect with respect to the medium adjacent the seal but produces a constant suction in the area externally preceding the seal. Consequently foreign matter can be drawn in with air into the seam of the seal causing increased wear and eventually premature failure of the seal. The preliminary seal is also subjected to considerable abrasion. Its useful life, and hence that of the seal, are accordingly not very satisfactory.
Ferrofluidic seals are known in the art and are described, for example in the German Patent publication (DOS) No. 35 01 937. Such seals make use of a permanent magnet that surrounds the shaft with a radial gap, and a ferrofluid disposed in the gap. These seals are used as main seals, and their ability to withstand pressure permits their use in cases in which the one side is exposed to atmospheric pressure and the other to a low partial pressure.